1. Field of the Invention
The invention relates to the production of hollow semiconductor bodies and somewhat more particularly to directly heatable semiconductor bodies and a method of producing the same.
2. Prior Art
In diffusion doping of semiconductor crystals, quartz tubes or ampules were generally used as a container for the crystals during the diffusion process, which requires relatively high temperatures in a suitable furnace or oven. However, quartz tubes or ampules are disadvantageous in the same manner as graphite tubes or ampules, i.e., it is necessary to avoid contact between the semiconductor crystal discs being doped and such diffusion containers. In addition, quartz diffusion containers require special diffusion ovens since quartz cannot be heated directly or by induction.
German Letters Patent No. 1,805,970 teaches a diffusion container composed of a semiconductor material instead of quartz or graphite. Such semiconductor diffusion containers are formed by depositing a select semiconductor material from a gaseous phase containing a thermally decomposable gaseous compound which yields the desired semiconductor material onto the outer surface of a heated carrier member or mandrel whereby a desired semiconductor tube or the like is formed. Thereafter, the mandrel is removed without destruction of the so-formed tube. Semiconductor tubes are able to withstand much higher temperatures than, for example, quartz or graphite tubes and accordingly allow diffusion processes utilizing such tubes to be greatly accelerated. Further, the crystal discs which are being doped can contact the semiconductor tube walls without any disadvantageous consequences.
Generally, semiconductor tubes of this type may be used as diffusion ovens. The ends of such a tube are sealed with appropriate semiconductor plugs having gas inlets and outlets therein so that gaseous doping materials, which may be mixed with a carrier gas, can be fed into the interior of the tube for diffusion into the semiconductor crystal discs therein. A heating coil may be provided about such a tube and supplied with electrical energy to heat the tube to the necessary diffusion temperatures by radiation heat. Alternatively, the coil may be provided with a high frequency energy source for induction heating of the tube. However, such a coil has a limited life expectancy and tends to give up heavy metal ions therefrom to the tube during coil operation, which may considerably impair the characteristics of the semiconductor crystals being doped.
German Offenlegungsschrift No. 1,933,128 suggests an arrangement for diffusion doping materials into semiconductor crystals wherein the diffusion container comprises a tube composed of a crystalline gas impermeable semiconductor material and which can be heated directly by a voltage source or by a high frequency source to a desired temperature. This tube may be provided with electrodes at the opposite ends thereof or it may be encompassed by an induction heating coil. In order to improve the heating of the tube during operation, a ring of a fairly conductive material, for example, graphite, is mounted about the tube. When the tube is heated directly by a voltage source, the voltage required to reach the necessary diffusion temperature is dependent on the dimension of the tube and on the conductivity characteristics of the semiconductor material forming the tube. Accordingly, the aforesaid Offenlegungsschrift suggests the use of a highly doped semiconductor material which can be economically produced into a tube so that the voltage required for the induction heating can be relatively low. Once a certain temperature is achieved, the conductivity of the tube is no longer dependent on the dopant within the semiconductor material but is only primarily dependent on the tube dimensions.
Production of extremely high purity gas impermeable tubes composed of semiconductor materials, such as silicon or silicon carbide, has become possible with the aforementioned gas phase deposition process. Such high purity tubes can only be heated with a direct current after a pre-heating process. When a doped semiconductor material forms the tube, such pre-heating is not necessary, as stated in German Offenlegungsschrift No. 1,933,128 and the tube can be heated directly. However, with doped semiconductor tubes, one has to contend with undesirable reactions between the tube dopant and the semiconductor crystal being doped, generally by some other dopant.
German Patent application Ser. No. P 22 53 411 8 owned by the instant assignee, suggests that high purity directly heatable semiconductor diffusion containers be formed as two-layer tubes or the like. In accordance with this concept, a layer of a high purity semiconductor material is deposited on a heated mandrel and then a layer of a doped semiconductor material is deposited on the high purity semiconductor layer. After the finished tube is removed from the mandrel, it has an inner surface composed of a high semiconductor material and an outer surface composed of a doped semiconductor material. Aside from the complexities of providing proper gaseous atmospheres at the proper times, the foregoing process is somewhat disadvantageous in that the outer layer dopants must be carefully selected to insure that they diffuse very slowly through the tube material since otherwise they would contaminate the primary diffusion process occurring within the tube.